Showing papers by "Eberhard Kerscher published in 2008"

Increasing the fatigue limit of a high-strength bearing steel by thermomechanical treatment

Abstract: High-strength steels have a maximum of the fatigue limit at a certain tensile strength. The decrease of the fatigue limit at still higher tensile strength can be explained by the failure mechanism of high-strength steels which fail due to crack initiation at inclusions or other internal flaws. To increase the fatigue limit two different thermomechanical treatments (TMTs) are applied to the material after classical martensitic hardening and tempering. A TMT results in a decrease of the hardness of about 10%. Different cyclic loading during the TMT can either reduce or increase the fatigue limit. In order to describe and compare the different treatments an evaluation of the stress intensity factors arising at the critical inclusions was carried out. To take into account different numbers of cycles to failure in the calculation a modification of the calculation of the stress intensity factor according to Murakami was successfully developed.

Relationships between process, microstructure and properties of molded zirconia micro specimens

Abstract: Due to size effects the mechanical behavior of micro-components with dimensions in the range of some 100 μm and structure details of about 10 μm differs markedly from those of larger components. This is a crucial aspect for the design of micro-components for applications where demands for high strength are critical. The present study, which was performed in the frame of the Collaborative Research Centre 499 (SFB 499), approaches this issue by investigating the relationship between production process, microstructure and the mechanical properties of micro-specimens made from zirconia using two different feedstocks. The specimens were produced by a sintering process. The sintering temperature was varied between 1,300 and 1,500°C. Mechanical and tribological behavior of the specimens was determined by three-point bending tests as well as static and sliding friction tests, respectively. Properties derived from these tests were then correlated to the surface states in the specimens such as porosity, edge radius and roughness. The strength of the micro-specimens was found to be significantly influenced by these surface features. Whilst low porosity alone is not sufficient for high strength, notch effects resulting from pores as well as surface roughness can lower the strength. With increasing edge radius the strength of the material also increases. The porosity, edge radius and surface roughness were mathematically correlated with the strength to allow for a forecast. Within the SFB 499 feedstocks with specific properties were designed and reliable processes were developed to guarantee desirable surface roughness and porosity in the specimens. A characteristic bending strength of about 2,000 MPa is realizable in the micro-specimens within a good statistical reliability. The tribological tests revealed that the wear properties of the zirconia micro-components are strongly dependent on the quality of the feedstock.

Thermal, mechanical and fretting fatigue of silicon nitride

Abstract: Tools for hot wire rolling are exposed to high thermal, mechanical and tribological loadings during service. Ceramic materials such as silicon nitride possess good mechanical and tribological properties up to temperatures where most of today' roller materials do not exhibit acceptable reliability. In the present paper basic mechanical properties under thermal and mechanical loading as well as the damage behaviour of Si3N4 have been investigated in high cycle thermal fatigue and dynamic as well as cyclic four point bending tests. In fretting fatigue tests the behaviour of Si3N4 under near-service conditions, as they appear during application in the hot wire rolling process, has been analysed. It is shown that in the temperature range investigated pure thermally induced loading leads to the same damage mechanism as pure mechanical loading and therefore thermally induced stresses can be directly added to any mechanical loading. The isothermal four point bending tests confirm the existence of a cycli...

Prototype Manufacturing of Extruded Aluminum Aircraft Stringer Profiles with Continuous Reinforcement

Abstract: For an increase in safety against crack initiation and growth in metallic structures of airplanes different concepts were developed in the past. In the focus of this work are profiles made of continuously reinforced extruded aluminum. The production and the used die set of these profiles is presented as well as problems occurring in terms of geometrical inaccuracies of the embedded high strength wires. In addition, this paper attends to the problem of lateral seam weld formation. The interface between the AA-2099 as well as AA-6056 aluminum alloy and the high strength wires Nivaflex and Nanoflex were characterized by metallurgic investigations and push-out tests. As a result it can be stated that a sufficient geometrical accuracy could be achieved and a high interface strength can be accomplished even if a slight gap is still present in the interface layer between matrix and reinforcing element.

Documentation of the Corrosion of Composite-Extruded Aluminium Matrix Extrusions using the Push-Out Test

Abstract: A possibility to increase both stiffness and strength of aluminium-based structures for the application in lightweight profiles for vehicle space frames is the use of composite extrusions in which high-strength metallic reinforcements are incorporated. Within the scope of the present investigations, composite-extruded profiles with wire-reinforcements made of austenitic spring steel 1.4310 (X10CrNi18-8), in an aluminium matrix AA6060 (AlMgSi0.5), which were exposed to different corrosive media for different times, were characterised in terms of the debonding shear strength using the push-out-technique. The formation of a galvanic couple could be conceived mathematically in regard of terms describing the formation of a shear-impeding layer and the corrosive attack. Thereby the parameters for the different media could be determined.

Mechanical Properties of Primary Shaped Notched Micro Specimens Made of Aluminum Bronze

Abstract: Tensile test micro specimens (gauge section 780 x 260 x 130 μm 3 ) made of aluminum bronze were produced by vacuum pressure casting. Two semicircular notches of radii ranging between 5 and 45 μm were located in the middle of the gauge length. These were tested under quasi static tensile conditions with a micro universal testing device. Results show that micro specimens are as sensitive to notch effect as macroscopic specimens.

Gefüge und Eigenschaften verbundextrudierter Aluminiumlegierungen

Abstract: Kurzfassung Strangpressen ermoglicht die flexible Fertigung von leichten Tragwerkstrukturen. Die Steifigkeit und Festigkeit sind dabei durch den eingesetzten Werkstoff, meist Aluminium oder Magnesium, sowie durch die Profilgeometrie festgelegt. Durch das Einbringen von hochfesten oder steifen Verstarkungselementen direkt im Strangpressprozess lasst sich die Leistungsfahigkeit der Tragwerkstruktur bei gleich bleibender Geometrie steigern. Die Veroffentlichung gibt einen Uberblick uber verschiedene im Verbundstrangpressen verarbeitbare Materialkombinationen, die dabei entstehenden Mikrostrukturen und mechanischen Eigenschaften. Dabei wird vor allem die Grenzflache zwischen Matrix und Verstarkungselement metallographisch und mit Push-out-Versuchen mechanisch charakterisiert. Mit Zugversuchen sowie zyklischen Tests wird das Versagensverhalten des Verbundes analysiert. Bisher werden Steigerungen der spezifischen Festigkeit von bis zu 80% erreicht.

Gefüge hybrider Verbundstrangpressprofile auf Aluminiumbasis

Abstract: Given the background of proven improvement possibilities in the mechanical properties of extrusion profiles through composite extrusion of metallic matrix materials with metallic reinforcing elements, a question arises as to the potential of hybrid composite extrusion using metallic and non-metallic components. In this paper, hybrid extruded profiles based on the aluminum alloy EN AW-6060, reinforced with a composite wire of 60 volume-% of Nextel aluminum oxide fibers and 40 volume-% of EN AW-1099, as well as based on the composite matrix; material EN AW-6061, inititally reinforced with 15 volume-% Al 2 O 3 particles, reinforced during composite extrusion with a spring steel wire, were manufactured and characterized metallographically. By way of a specially constructed extrusion apparatus, operated on a universal testing machine, test samples of 20 mm length were produced at extrusion temperatures of 450° C. The subsequent characterization in light-optical and scanning electron microscopes shows a gapfree embedding of the wires in the matrix materials which indicates a good wire/matrix bonding. The assumption is that the hybrid extrusion shows a marked improvement ofthe mechanical properties compared to non-hybrid profiles. No fracturing of the wire reinforcements occurred during the extrusion process. Very high extrusion forces are necessary for the production of hybrid extrusions particularly during usage of composite matrices it as has been shown for composite extrusion of metallic components. In addition, usage of reinforced matrices will result in high wear of tools.